Restored pre-r10062 behavior allowing None from formfield_callback to exclude itself from the form

    in the ``fields`` argument.

    """

    field_list = []

    ignored = []

    opts = model._meta

    for f in opts.fields + opts.many_to_many:

        if not f.editable:

        formfield = formfield_callback(f, **kwargs)

        if formfield:

            field_list.append((f.name, formfield))

        else:

            ignored.append(f.name)

    field_dict = SortedDict(field_list)

    if fields:

        field_dict = SortedDict([(f, field_dict.get(f)) for f in fields if (not exclude) or (exclude and f not in exclude)])

        field_dict = SortedDict(

            [(f, field_dict.get(f)) for f in fields

                if ((not exclude) or (exclude and f not in exclude)) and (f not in ignored)]

        )

    return field_dict

class ModelFormOptions(object):

    def __unicode__(self):

        return unicode(self.biggie)

class MarkupField(models.CharField):

    def __init__(self, *args, **kwargs):

        kwargs["max_length"] = 20

        super(MarkupField, self).__init__(*args, **kwargs)

    def formfield(self, **kwargs):

        # don't allow this field to be used in form (real use-case might be

        # that you know the markup will always be X, but it is among an app

        # that allows the user to say it could be something else)

        # regressed at r10062

        return None

class CustomFieldForExclusionModel(models.Model):

    name = models.CharField(max_length=10)

    markup = MarkupField()

__test__ = {'API_TESTS': """

>>> from django import forms

>>> from django.forms.models import ModelForm, model_to_dict

<tr><th><label for="id_description">Description:</label></th><td><input type="text" name="description" id="id_description" /></td></tr>

<tr><th><label for="id_url">The URL:</label></th><td><input id="id_url" type="text" name="url" maxlength="40" /></td></tr>

# Model field that returns None to exclude itself with explicit fields ########

>>> class CustomFieldForExclusionForm(ModelForm):

...     class Meta:

...         model = CustomFieldForExclusionModel

...         fields = ['name', 'markup']

>>> CustomFieldForExclusionForm.base_fields.keys()

['name']

>>> print CustomFieldForExclusionForm()

<tr><th><label for="id_name">Name:</label></th><td><input id="id_name" type="text" name="name" maxlength="10" /></td></tr>

# Clean up

>>> import shutil

>>> shutil.rmtree(temp_storage_dir)